In recent years, there has been an increasing demand from the aerospace industry and other industrial applications for epoxy materials having high temperature utility. High temperature utility can be improved through the use of anhydrides and aromatic amine curing agents as well as through the use of epoxy resin obtained by the eposidation, with peroxy compounds, of double bonds in certain Diels - Alder adducts. However, in many applications the high temperature curing is prohibitive. Studies indicate that temperature resistance is a function of crosslinking density of the cured resin, with higher crosslinking affording improvements in these properties. Higher crosslink density can be achieved by increasing the functionality of either the epoxy resin or the hardening agent. Thus there is a continuing search for new epoxy resins and hardening agents which can afford improvements in the properties of the cured materials.
Organic acid dianhydrides which contain either cyclic or aromatic structures and have high functionality have been found to impart improved heat resistance, as well as increased chemical and solvent resistance to cured epoxy resin compositions. Among such dianhydrides are pyromellitic dianhydride, cyclopentadiene dianhydride, and benzophenone tetracarboxylic dianhydride. The organic acid dianhydrides, while effective in imparting improved properties to cured epoxy resin compositions, suffer from the drawback that they are generally high melting solids which are not soluble in common solvents, nor epoxy resins, except at elevated temperature, and therefore curing at ambient temperature is not possible.
It has been discovered (J. A. Graham, VSP, 4,002,599 1977, see also J. A. Graham and J. E. O'Connor, Adhesive Age, July 1978 p. 20-23), that biphenyl anhydride epoxy curing agents are solubilized at ambient temperature in polyglycidyl derivatives of aminophenols to afford epoxy resin compositions curable at room temperature to provide epoxy systems characterized by high temperature resistance with a tensile lap shear at RT of about 2000 psi, and at 300.degree. F. about 1000 psi, with no peel data being provided.